1. Field of the Invention
The invention concerns a method for measuring a fluid flow rate between two points spaced in the direction of a fluid flow according to which the flow rate is obtained by combining a measurement of the propagation times of two acoustic signals transmitted between the two points in opposing directions with a measurement of the acoustic phase shifts induced in each acoustic signal via the propagation of each of said acoustic signals in the flow.
2. DESCRIPTION OF THE RELATED ART
For a large number of years, the flow rate of a fluid (or its volume) flowing in a pipe is measured by using the propagation of acoustic signals transmitted between two acoustic transducers situated at points spaced in the direction of the flow of the fluid.
In principle, an acoustic signal transmitted from the first transducer to the second transducer is received by this second transducer and the propagation time T1 of this acoustic signal is measured.
Similarly, the propagation time T2 of an acoustic signal transmitted from the second transducer to the first transducer is measured after said signal has been received by this first transducer.
The flow rate of the fluid Q in a pipe can then be written as follows: EQU Q=(SL/2)(T2-T1)/T1T2,
where S denotes the mean passage section available to the flow between the two acoustic transducers and L denotes the distance separating the transducers.
Now, the precise measurement of the propagation time of an acoustic signal can be relatively complex to implement, this depending on the accuracy sought.
For these reasons, the phase measuring method is preferable to the method for solely measuring the propagation time as the phase measurement is able to more simply obtain the desired precision concerning the measurement of the flow rate. The European patent application no 0426309 moreover describes a method for measuring the flow rate of a fluid and able to be used in a fluid counter and combining a measurement of the propagation times of two acoustic signals transmitted between two points spaced in the direction of the flow of the fluid in opposing directions with a measurement of the acoustic phase shifts induced in each acoustic signal via the propagation of each of said acoustic signals in the flow.
The flow rate of the fluid Q in a pipe can be written as follows: EQU Q=(SL/4.pi.Fac)(2.pi.[Fac(T2-T1)]+(.phi.2-.phi.1))/T1T2,
where T1 and T2 respectively represent the propagation times of an acoustic signal transmitted in a downstream direction and in the upstream direction of the flow of the fluid, .phi.1 and .phi.2 respectively the acoustic phase shifts induced in each of the acoustic signals on account of the propagation of these signals and Fac represents the frequency of said acoustic signals.
This expression reveals a first term, 2.pi.[Fac(T2-T1)], which determines the number of whole periods existing in the difference of the propagation times. The expression [x] denotes the entire portion of x. A second term, .phi.2-.phi.1, determines the precise phase shift between -2.pi. and 2.pi. which affects the acoustic signal at the time it propagates and which is due to the presence of a fluid flow rate between the transducers. This second term makes it possible to refine the measurement and thus obtain a more precise flow rate measurement. The quantity T1T2, which appears to the denominator of the formula of the flow rate, corresponds to the square of the mean propagation time of the acoustic waves transmitted in the upstream and downstream directions.
Given the fact that T1=L/c-v and T2=L/c+v, where c and v respectively represent the propagation speed of the acoustic signal and the speed of the fluid, and as the propagation speed of the signal mainly depends on the temperature which varies slightly during the flow rate measurements, the term T1T2 varies slightly for a given fluid composition and accordingly this term is not recalculated on each new measurement.
In this document, a first measurement of a propagation time of an acoustic signal is carried out by inverting the phase of a specific location of the transmission signal and by detecting on the signal received the moment corresponding to this phase inversion. The detection of this moment is effected with the aid of an instantaneous phase detector. This measurement carried out in the upstream and downstream directions provides the first term of the expression indicated above: 2.pi.[Fac(T2-T1)].
After having sampled the signal receiver in eight capacitors and digitally converted said sampled signal, the measurement of the acoustic phase shift is effected by carrying out a synchronous detection of this digitalized signal, which makes it possible to determine the phase shift .phi.1 or .phi.2 which is interpreted as the phase difference between the phase of the sampled signal and the phase of the reference signal.
After having determined the acoustic phase shifts corresponding to the downstream .phi.1 and upstream .phi.2 directions according to this method, the term .phi.2-.phi.1 is determined by the difference.
Thus, by adding the two terms 2.pi.[Fac(T2-T1)] and .phi.2-.phi.1, the fluid flow rate is determined more accurately than previously.
However, in certain applications where the energy consumption needs to be as small as possible, especially because the energy source is a battery and its period of life is limited, it is essential to reduce this consumption to a minimum.
In addition, this method for determining the fluid flow rate is rather complicated to implement and requires significant digital calculation volumes.